Feature: Implements the treat-unmeasured-as-uncovered flag

[aweidner]

It's possible for coverage files to omit lines from a diff entirely rather than indicating that they were not hit. This pull request introduces a new flag to allow optionally counting these occurrences towards uncovered lines.

Current behavior

This diff for example

diff --git a/some_file.py b/some_file.py
index b9f1ea0b4e..6de5313ab7 100644
--- a/some_file.py
+++ b/some_file.py
@@ -606,3 +606,6 @@ urlpatterns = [
         name="redirect_to_catalog",
     ),
 ]
+
+def uncalled_func():
+    print("Hello World")

Along with this fragment of coverage in the lcov xml format:

...
<lines ...>
    <line number="610" hits="1"/>
</lines>
...

In this example, line 611 appears in the diff, but does not appear at all in the coverage.xml file. When diff-cover is run against this combination, we get the following output

-------------
Diff Coverage
Diff: origin/main...HEAD, staged and unstaged changes
-------------
some_file.py (100%)
-------------
Total:   1 line
Missing: 0 lines
Coverage: 100%
-------------

Because only one line is in common between the diff and the coverage information. With the existing behavior, the xml file would need to contain hits="0" lines as such:

<lines ...>
    <line number="610" hits="1"/>
    <line number="611" hits="0"/>
</lines>

In order to detect all of the changes in the diff, not just the ones that were executed.

New behavior

This pull request proposes a new flag --treat-unmeasured-as-uncovered (I'm open to renaming this). When set, this flag detects any unmentioned non-whitespace lines in the diff as being uncovered and reports them. For the example above, the output is:

-------------
Diff Coverage
Diff: origin/main...HEAD, staged and unstaged changes
-------------
some_file.py (50.0%): Missing lines 611
-------------
Total:   2 lines
Missing: 1 line
Coverage: 50%
-------------

Existing behavior is unchanged when the flag is off, preserving the old functionality for anyone who does not opt-in.